Freelancing and Taxes

I am beginning to look into doing jobs for money, and being a newbie as I am into the freelance world I have a few questions about Freelancing and the IRS.

Is there an income point where when you exceed that point you must begin paying taxes? (ex. If Joe only made $325 this year doing freelance work does he have to file for taxes)

Receipts: How do you record your expenditures? What do and don't you count as expenditures? What kind of documents do you have, or collect to prove your earnings?

Do you need to register for any legal issues of any kind if you're operating by yourself out of your home?

Thank you.